Fuel supply method for direct methanol fuel cell

ABSTRACT

In a direct methanol fuel cell, fuel efficiency is maintained by periodically adding a higher methanol concentration mixture through a cartridge into the primary fuel container. The cartridge replenishes methanol and partial water losses due to the consumption of fuel in the power generating process. In a typical system, the fuel replenishment mechanism is controlled through an electronic apparatus that monitors the power conversion process and is capable of predicting remaining operating capacity.

This application claims priority from U.S. appl. No. 60/440,657 filedJan. 15, 2003, which application is incorporated herein by reference forall purposes.

The invention relates generally to fuel cells and methods for use withfuel cells, and relates more particularly with direct methane fuelcells.

BACKGROUND

Most handheld consumer electronic devices, such as wireless telephones,notebook computers, and personal digital assistants (PDAs) are poweredeither by rechargeable batteries or by disposable batteries.

In the area of rechargeable batteries, historically nickel-cadmiumbatteries were used. More recently nickel-metal-hydride has been usedand still more recently lithium-ion technology has been used. Theseshifts in battery chemistry have improved the power-to-weight ratio butnature imposes upper bounds on the energy density available inrechargeable batteries.

In the area of disposable batteries the chief technology employedpresently is alkaline cells. Nature also imposes upper bounds on theavailable energy density for such cells.

Those who design handheld consumer electronic devices are thus facedwith limits on battery life imposed in part by a desire to keep thedevices from getting too heavy and large.

In recent years much attention has been paid to the prospect ofemploying fuel cells in a variety of applications including the poweringof handheld consumer electronic devices. It seems possible that aftervarious challenges are overcome, fuel cells may prove to be a usefulpower source for such applications. Fuel cells offer the possibility ofa light-weight power source using inexpensive fuel, with fuel that iseasy to refill.

There are, however, a number of challenges with present-day fuel cells.They run down. Refilling them can be a bother. It is not easy to extractall available energy from a given charge of fuel.

One fuel cell is called a direct methanol fuel cell (DMFC). In a DMFC,methanol is reacted with oxygen, one byproduct of which is water. Asshown in FIG. 3, the methanol and oxygen flow toward a proton exchangemembrane. Electrical power is derived from an anode and cathodejuxtaposed with the membrane. Importantly, the “active fuel” area 2 inFIG. 3 is not filled with pure methanol but instead contains a solutionof methanol in water. A typical methanol concentration is 3%.

DFMCs generate electricity through decomposition of methanol intohydrogen ions and electrons. Hydrogen ions propagate through the protonexchange membrane into the cathode area, while electrons reach thecathode through a load providing electricity in the process. Electronsreaching the cathode area recombine with hydrogen ions which in turncombine with oxygen supplied by the air to provide pure water as abyproduct.

Many investigators have attempted to devise suitable fuel cellstructures, as shown for example in U.S. published applicationpublication No. 20040001989 entitled “Fuel reservoir for liquid fuelcells” and publication No. 20020127141 entitled “Multiple-walled fuelcontainer and delivery system.” See also published internationalapplication with publication No. WO 03/094318 entitled “Device andmethod to expand operating range of a fuel cell stack.”

As mentioned above, DMFCs are comparatively small and potentiallysuitable to be used in small electronic appliances. Key for such anadoption is further miniaturization, reduction in cost as well asimprovements in the performance of the cell.

It would be extremely desirable to devise ways to improve DMFCs toperform better.

SUMMARY OF THE INVENTION

This invention addresses some of the key performance issues relating tothe effectiveness of the fuel usage as well as the ease of use by theconsumer and the integration of the technology with power managementmethods employed in portable computer and electronic devices. Towardthis end a description is provided of the fuel storage apparatus toimplement a fuel replenishment method. A description is provided of anapparatus to store a high-concentration methanol mixture (describedhereon as cartridge). A mechanism is described that controls the openingand closure of the passage between the cartridge and the main fuelcontainer in proportion to (or in response to) the output voltage of thecell.

An alternative mechanism is described that adjusts the methanolconcentration in the main fuel container based on calculations of theremaining capacity of the cell. A manual method is described that can beactuated by the user to force replenishment when there is not enoughpower left to drive the automatic replenishment. A method is describedto calculate remaining electric energy in the cell based on electriccurrent (load current) and cell voltage measurements (See FIG. 1). Amongother things this makes it easier to display the remaining capacity of acell (e.g. a “smart cell”). It is possible to adjust the fuelconcentration based on information provided by the electronics regardingthe remaining fuel capacity.

In a direct methanol fuel cell, fuel efficiency is maintained byperiodically adding a higher methanol concentration mixture through acartridge into the primary fuel container. The cartridge replenishesmethanol and partial water losses due to the consumption of fuel in thepower generating process. In a typical system, the fuel replenishmentmechanism is controlled through an electronic apparatus that monitorsthe power conversion process and is capable of predicting remainingoperating capacity.

In situations where the cell voltage decreases below a criticalthreshold, (as determined from the cell output voltage), the destructivepiercing of the cartridge is described in order to recover celloperation.

DESCRIPTION OF THE DRAWING

The invention is described with respect to a drawing in several figures.

FIG. 1 shows voltage and current for a typical DMFC.

FIG. 2 shows voltage and current for a DMFC in which stirring isemployed.

FIG. 3 shows a DMFC using a recharging cartridge and a pushing pin.

FIG. 4 shows a DMFC using a recharging cartridge and a mini pump.

FIG. 5 shows the DMFC of FIG. 4 with the pump in operation.

FIG. 6 shows voltage and current for a DMFC in which more fuel isinjected.

DETAILED DESCRIPTION

FIG. 1 shows voltage and current for a typical DMFC. The horizontal axisin this figure (and in FIGS. 2 and 6) is time in arbitrary units. The Yaxis (in these three figures) shows voltage and current. In FIG. 1 aplot 31 shows a degradation of output voltage into a standard load. Plot32 shows the available current. An abrupt event defining an operatingstop voltage occurs at time 33, after which voltage is very low and oflimited use. A small motor was used as a test load for the cells shownin these plots.

Importantly, the product of voltage and current can be integrated overtime, defining a watt-hour capacity as shown in the figure. Thiswatt-hour capacity is one of the important parameters that one seeks tooptimize when working with fuel cells.

FIG. 2 illustrates the effect of stirring the fuel mixture in theperformance of the cell. The effect of stirring is attributed by theauthors to the fact that concentration of methanol in the anode area isconstantly reduced because of its decomposition into hydrogen and carbondioxide. Stirring achieves homogenous concentration in the containerincluding the anode area. In FIG. 2, the results without stirring may beseen with voltage line 44 and current line 42. With stirring, theresults are seen with voltage line 43 and current line 41. As will beappreciated, the total power generated (defined by the product ofvoltage and current, integrated over the production interval) is muchgreater with stirring than without.

The conclusion is that even when the fuel has originally optimalmethanol concentration it cannot maintain it in the anode area withoutsuch assistance.

In one embodiment of the invention, then, a piezo-pump (or other stirrermeans) is provided to circulate the fuel in order to provide constantlya maximum or nearly maximum possible methanol concentration in the anodearea. Experience shows that it may not be needed to stir continuously.Instead, the stirring can be done at intervals. The frequency of thestirring depends on the size and the shape of the main fuel container.The stirrer may be a piezo pump such as that described below.

FIG. 3 presents another embodiment of the invention. In this embodiment,the prior-art parts of a DMFC can be seen—a fuel container 1, an activefuel location 2, a proton exchange membrane, an oxygen inlet, and ananode and cathode. Importantly, what is also provided is a cartridge 3containing a high concentration methanol mixture, a valve 6, a pushingpin 8, and a safety lock 7. The safety lock 7 serves to preventinadvertent pushing of the pushing pin 8.

When a critical threshold is determined either by the cell voltage or bycalculating the remaining electrical power, the system issues an alertto the user. The user subsequently removes the safely lock 7 and pushespushing pin 8. As a result of this action a needle 5 pierces thecartridge 3 enabling the mixture of high methanol concentration fluid 4into the main container 1. As a result, the enriched fuel recovers theoperation of the cell.

This method can be effective in implementations with a relatively smallfuel canister, such as cellular phones. Using this method a singlecartridge can double the power-generating ability of the cell.

FIGS. 4 and 5 present other embodiments of this invention. When theelectronic apparatus (omitted for clarity in FIGS. 4 and 5) detects acritical low condition (either by voltage measurement of by remainingcapacity calculations) it actuates a mini-pump 7, forcing a determinedamount of high-methanol-concentration fluid from the cartridge 3 intothe main fuel container 1. The fluid passes through a valve 5. The minipump 7 contains a piezo device 6 which is driven by wiring 9 by a drivecircuit, omitted for clarity in FIG. 4.

FIG. 5 highlights the operation of the mini-pump. In a typicalarrangement a microcontroller 22 emits a pulse train 21 which moves thepiezo device 6 back and forth. This pumps the fluid as shown by arrows23.

A microcontroller 22 calculates the cell's remaining capacity based oncell voltage, temperature, etc., and determines the amount of time thepump 7 has to be activated each time.

The microcontrolle 22 causes the piezo element 6 to vibrate for aspecified amount of time restoring the methanol concentration in themain container 1.

The duty cycle of this operation is adjusted dynamically determined uponthe volume of the main container 1 and the calculated fuel concentrationat each time.

FIG. 6 shows actual measurements of a fuel cell benefitting from thisinvention. Plots 51 and 52 show voltage and current respectively.Seventeen hours pass during which power is readily available from thefuel cell. Then, before an operating stop voltage (see FIG. 1) isreached, at point A there is an addition of fuel. In this particularcase 2 cubic centimeters (ccs) of a fluid comprising a 10% concentrationof methanol is added to the fuel container 1. Stated differently, atpoint A, just prior to the motor stopping, 2 cc of a 10% methanolsolution was added and stirred into the remaining fluid of the maincontainer.

As may be seen, the voltage returns to a previous high level, as doesthe current through the standard load. The life of the cell is extendedto more than thirty-three hours, nearly doubling the service life of thecell.

It should be noted that the determination of point A depends on theapplication and the load conditions. In addition the amount of fuelinjection has to be determined accurately based on the volume of fluidin the main container, so the active fuel never exceeds the optimumconcentration of 3%. The reason for this limit is that a higher-than-3%concentration of active fuel will cause rapid deterioration in theplatinum used as a catalyst as well as in the proton exchange membrane.

It will be appreciated that if a different catalyst or a differentproton exchange membrane were employed the limit on concentration mightbe different.

The method described above prolongs the operation of the fuel cell whilepreserving at the same time the useful life of the cell itself.

Looking at FIG. 6 we observe that the voltage of the cell recoversrapidly. This would not be the case if the replenishment fluid were notactively stirred into the main container 1.

In one embodiment, the DMFC fuel replenishment container (cartridge) 3can be discarded after it is used in order to be replaced by a newcartridge 3. It is anticipated that this fuel replenishment method isthe most convenient for the user, as it does not involve direct usercontact with chemicals in order to refill the cell. One way to proceedin accordance with the invention is to provide cartridges with thecorrect methanol concentration to be used for a single load cycle.

This invention enables the use of the same cartridge for a much longertime, increasing the usage of the cell as well as reducing the cost ofoperating it.

Given the methanol concentration (solution of 3% typically) and thevolume of the fuel, a theoretical calculation of the quantity ofelectricity that can be produced from a DMFC can be performed. Theactual performance of the cell however is affected by dynamic parameterssuch as cell temperature, ambient temperature, load condition, etc.Effective determination of the cell's capacity can be estimatedaccurately by taking into consideration all these operating parametersalong with the fuel condition.

It should be noted that in specific mobile applications such as cellularphones it may not be necessary to implement the micro pump mechanismbecause stirring occurs naturally as the user moves the equipment.

Those skilled in the art will have no difficulty devising myriad obviousimprovements and variations upon the invention, all of which areintended to be within the scope of the claims which follow. Among otherthings, it is expected that the teachings of the invention benefit fuelcells other than DMFCs.

1. Direct methanol fuel cell apparatus comprising: a fuel container; ananode adjacent the fuel container; a proton exchange membrane adjacentthe anode; a cathode adjacent the proton exchange membrane; an oxygensupply adjacent the cathode; the fuel container containing methanol inwater at a first concentration; a cartridge selectively communicativelycoupled with the fuel container; the cartridge containing fluidcomprising methanol in water at a second concentration, the secondconcentration higher than the first concentration.
 2. The apparatus ofclaim 1 wherein the second concentration is at least double the firstconcentration.
 3. The apparatus of claim 2 wherein the secondconcentration is at least triple the first concentration.
 4. Theapparatus of claim 1 wherein the selective communicative couplingcomprises a pushing pin actuable by a human user, said pin puncturingthe cartridge.
 5. The apparatus of claim 1 wherein the selectivecommunicative coupling comprises a pump actuable by electronic means,said pump pumping fluid from the cartridge to the container.
 6. A methodfor use with a direct methanol fuel cell, the method comprising thesteps of: bringing a first solution of methanol in water at a firstconcentration into contact with an anode, the first solution containedwithin a container; bringing oxygen into contact with a cathode, thecathode adjacent a proton exchange membrane and the proton exchangemembrane adjacent the anode; at a later time, bringing a cartridge intocommunicative coupling with the container, the cartridge containing asecond solution of methanol in water at a second concentration, thesecond concentration higher than the first concentration.
 7. The methodof claim 6 wherein the second concentration is at least double the firstconcentration.
 8. The method of claim 7 wherein the second concentrationis at least triple the first concentration.
 9. The method of claim 6wherein the step of bringing the cartridge into communicative couplingwith the container comprises a human user pushing a pin, said pinpuncturing the cartridge.
 10. The method of claim 6 wherein the step ofbringing the cartridge into communicative coupling with the containercomprises actuating a pump, said pump pumping fluid from the cartridgeto the container.
 11. Direct methanol fuel cell apparatus comprising: afuel container; an anode adjacent the fuel container; a proton exchangemembrane adjacent the anode; a cathode adjacent the proton exchangemembrane; an oxygen supply adjacent the cathode; the fuel containercontaining methanol in water; and a stirrer within the fuel container.12. The apparatus of claim 11 further comprising electronics operatingthe stirrer at intervals as a function of measurements made regardingthe fuel cell apparatus.
 13. A method for use with a direct methanolfuel cell, the method comprising the steps of: bringing a solution ofmethanol in water into contact with an anode, the solution containedwithin a container; bringing oxygen into contact with a cathode, thecathode adjacent a proton exchange membrane and the proton exchangemembrane adjacent the anode; at a later time, stirring the solution. 14.The method of claim 13 wherein the stirring occurs as a result of ahuman user moving the fuel cell while it is in use.
 15. The method ofclaim 13 wherein the stirring occurs as a result of a stirring by astirrer contained within the container.